Bad o2 sensor?
There's quite a bit of misinformation on how to look at O2 sensor values..
Understand that this is heavily dependent on how close your fuel trims are to being '0' ina regular scantool or '100' (In Evoscan)
If your fuel trims are over 20% off, either +20 or -20 (in a scantool that reads 0% for perfect trims) or 30-40% in Evoscan you will have problems..
You will frequently throw a rich or lean code, and the ECU will swing wildly back and forth trying to find a stoich that it might not be able to resolve.
Typially oversized injectors and scaling using only the fuel tables can be a factor, other times venting at idle without recirculating, and other types of airflow changes from stock can alter it.
Now, if the 02 voltage number bounces around alot at idle and part throttle driving, thats pretty normal as long as your short term and long term trims are within 10%
On your wideband if its calibrated correctly, it will hover around 14.7, but it will swing back and forth from 15.0 to 13.0 depending on your mods and how airflow catches up or slows down with respect to the engine speed..
On acceleration airflow lags behind engine demand, therefore there is a setting for "tip-in" or acceleration, this value acts as an "Accelerator pump" similar to a carburetor where sudden changes in throttleinput will add fuel to prevent a lean tip-in stumble. If you do it very slowly, you may not get this to kick in and it will "tip in lean" as you transition from a standstill to very slow speeds at low rpm.
Now, what does all of this mean?
Well, first you need to be certain that the ECU is running in closed loop, from the factory, with factory injectors, and no major alterations to the volumetric efficiency of the engine, the low load portions of the map should run around 14.7 even in open loop.. Altering things changes that. So you need to be certain the car is actually operating in closed loop and your trims aren't completely out of whack.
If its operating correctly in closed loop, and your fuel trims aren't showing a value +/-20% or more from 0... Then the car will be generally around 14.7.. If the trims are way off, then you might have a bad O2 sensor, or some modification you made forced the trims so far out of adjustment that the ECU is out of range.. Then you can see AFR"s of 13.0 or so, this is usually accompanied by a CEL for too rich or too lean.
If everything works fine, and your trims are okay, but your wideband is reading incorrectly, then the Wideband could need to be 'free air calibrated' or the sensor is damaged.. When you decelerate fuel injectors cut fuel and should fairly quickly register the LEANEST value your wideband will register, or OL (which means over limit)
If the value is about 13.0 and stays there no matter what, then the 02 sensor is likely damaged since a calibration issue would still read an out of range lean condition.
Understand that this is heavily dependent on how close your fuel trims are to being '0' ina regular scantool or '100' (In Evoscan)
If your fuel trims are over 20% off, either +20 or -20 (in a scantool that reads 0% for perfect trims) or 30-40% in Evoscan you will have problems..
You will frequently throw a rich or lean code, and the ECU will swing wildly back and forth trying to find a stoich that it might not be able to resolve.
Typially oversized injectors and scaling using only the fuel tables can be a factor, other times venting at idle without recirculating, and other types of airflow changes from stock can alter it.
Now, if the 02 voltage number bounces around alot at idle and part throttle driving, thats pretty normal as long as your short term and long term trims are within 10%
On your wideband if its calibrated correctly, it will hover around 14.7, but it will swing back and forth from 15.0 to 13.0 depending on your mods and how airflow catches up or slows down with respect to the engine speed..
On acceleration airflow lags behind engine demand, therefore there is a setting for "tip-in" or acceleration, this value acts as an "Accelerator pump" similar to a carburetor where sudden changes in throttleinput will add fuel to prevent a lean tip-in stumble. If you do it very slowly, you may not get this to kick in and it will "tip in lean" as you transition from a standstill to very slow speeds at low rpm.
Now, what does all of this mean?
Well, first you need to be certain that the ECU is running in closed loop, from the factory, with factory injectors, and no major alterations to the volumetric efficiency of the engine, the low load portions of the map should run around 14.7 even in open loop.. Altering things changes that. So you need to be certain the car is actually operating in closed loop and your trims aren't completely out of whack.
If its operating correctly in closed loop, and your fuel trims aren't showing a value +/-20% or more from 0... Then the car will be generally around 14.7.. If the trims are way off, then you might have a bad O2 sensor, or some modification you made forced the trims so far out of adjustment that the ECU is out of range.. Then you can see AFR"s of 13.0 or so, this is usually accompanied by a CEL for too rich or too lean.
If everything works fine, and your trims are okay, but your wideband is reading incorrectly, then the Wideband could need to be 'free air calibrated' or the sensor is damaged.. When you decelerate fuel injectors cut fuel and should fairly quickly register the LEANEST value your wideband will register, or OL (which means over limit)
If the value is about 13.0 and stays there no matter what, then the 02 sensor is likely damaged since a calibration issue would still read an out of range lean condition.
Its REALLY REALLY REALLY important if you guys intend to tune your own cars that your Wideband sensor is working properly, but that the rest of closed loop operation is also working properly, since it can be the symptom of another issue that needs to be resolved before you can adequately tune the car.
Its really important to understand the basic principals of engine theory since it will help alot in these areas.
Its really important to understand the basic principals of engine theory since it will help alot in these areas.
/\ thats why I always go to malibujack for any technical questions as he always has insight on the subject matter...
For me, my readings were indicitative of a lead soaked front o2 sensor...
For me, my readings were indicitative of a lead soaked front o2 sensor...
Good thread, nice findings kmcconn9, I might try switching my front sensor in the near future, and MJ you are always good for the hardcore info...
FYI, i throw a lean condition code once in a while.
FYI, i throw a lean condition code once in a while.
Last edited by Nez136; Sep 26, 2006 at 10:22 AM.
Evolving Member
Joined: Dec 2006
Posts: 276
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Much like silicon from a gasket, etc., can get oxidized into a sensor destroying glass like coating inside the sensor.
-jjf
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